net/mlx5: Use effective affinity mask for IRQ selection

net/mlx5: Use effective affinity mask for IRQ selection

When a sf is created after a CPU has been taken offline, the IRQ pool may
contain IRQs with affinity masks that include the offline CPU. Since only
online CPUs should be considered for IRQ placement, cpumask_subset() check
would fail because the iter_mask contains offline CPUs that are not present
in req_mask, causing sf creation to fail.

This is an example:
  1. When mlx5 driver loads, it initializes the IRQ pools.
     For sf_ctrl_pool with ≤64 sf:
     - xa_num_irqs = {N, N} (There is only one slot)
  2. When the first SF is created:
     - The ctrl IRQ is allocated with mask=cpu_online_mask={0-191}
  2. We take CPU 20 offline
  3. Existing ctl irq still have mask={0-191}
  4. Create a new SF:
     - req_mask={0-19,21-191}
     - iter_mask={0-191}
     - {0-191} is NOT a subset of {0-19,21-191}
     - least_loaded_irq=NULL
  5. Try to allocate a new irq via irq_pool_request_irq()
  6. xa_alloc() fails because the pool is full(There is only one slot)
  7. sf creation fails with error

Use irq_get_effective_affinity_mask() instead, which returns the IRQ's
actual effective affinity that already excludes offline CPUs.

Fixes: 061f5b23588a ("net/mlx5: SF, Use all available cpu for setting cpu affinity")
Suggested-by: Shay Drory <shayd@nvidia.com>
Signed-off-by: Fushuai Wang <wangfushuai@baidu.com>
Reviewed-by: Shay Drory <shayd@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://patch.msgid.link/20260605102112.91772-1-fushuai.wang@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>